EVOLUTION OF MIOCENE MAGMATISM ALONG DEATH VALLEY, CALIFORNIA: ND-SR-PB ISOTOPIC CONSTRAINTS

Synextensional Miocene magmatism swept along the present Death Valley (DV) area from south to north at 12 to <6 Ma. High-level silicic plutonic rocks (Granite of Kingston Peak) as well as basalts and andesites in the Resting Springs Formation in southern DV were emplaced at ~12 Ma, silicic plutons (Shoshone, Little Chief stock) and the Black Mountain diorite in central DV yield isotopic ages of 11.6 to 8.7 Ma, and granitoids, diorites, basalts and andesites in the Last Chance Range and Little Sand Spring in northern DV are dated at 7.4 to 6 Ma. Our 20-sample whole-rock Nd-Sr-Pb isotope data set reveals important differences in the initial composition of these magmatic rocks. The e_Nd values range from -7.6 to -5.6 in southern DV, -6.3 to -1.9 in central DV, and -12.6 to -7.1 in northern DV. Sr_i values vary from 0.7065 to 0.7126 (southern DV), 0.7062 to 0.7080 (central DV), and 0.7067 to 0.7091 (northern DV). Measured ²⁰⁶Pb/²⁰⁴Pb range from 18.903 to 19.236 in southern DV, 18.743 to 18.821 in central DV, and 18.092 to 18.850 northern DV. In e_Nd vs. Sr_i space, magmatic rocks in the northern and central DV yield the least and most juvenile compositions, respectively. In ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb space, the southern group is the most and the northern group the least radiogenic. These observations suggest that Miocene subcrustal thermal perturbations and resultant bimodal magmatism in the DV area varied from south to north. Cratonic influence was most pronounced in northern DV, whereas the more depteted rocks of central DV may register the locus of magmatism that was syntectonic with Miocene extension of the DV region.